Reservoir characterization may be accomplished in a variety of ways for modeling behavior of fluids within a reservoir under different sets of circumstances and for finding optimal production techniques to maximize production. Seismic and microseismic surveys may be used for many applications, including for characterizing structure, lithology, fractures, and fluid distribution in a reservoir. One example of an application for seismic or microseismic surveying for fluid monitoring is in hydraulic fracturing operations, wherein microseismic monitoring can be used to track the propagation of a hydraulic fracture as it advances through a formation. Seismic events can be detected, located and used to detect propagation of fluids (or fractures). Software may provide modeling, survey design, microseismic events detection and location (which may be optionally automated), uncertainty analysis, data integration, and visualization for interpretation.
Seismic events are acoustic events generated by force, such as by airguns, vibroseis, dynamite, etc. Microseismic events are elastic events generated by rock movement. Microseismic events may be generated during hydraulic fracturing as well as during other operations, such as fluid production, water, gas or steam flooding, or formation compaction. In seismic surveys, a seismic source may induce seismic waves in the earth. In a microseismic survey, the sources of the seismicity/seismic waves or the like may be natural or induced fractures in the earth. In both cases, the seismic waves may propagate through the earth, be transmitted, reflected, and diffracted by formations or discontinuities, and can be detected by a plurality of sensors, at the surface or within the earth. Each of the sensor monitors the seismic wavefield, which is then recorded. The data received by a sensor, and then recorded, is collectively referred to as a trace. The collection of traces may be processed to gain information about the earth's subsurface, or stored for later processing.
Seismic and microseismic events, both naturally occurring and induced, can be characterized by a moment tensor that describes a unique radiation pattern, having a polarity component and an amplitude component, of the compressional and shear seismic energy radiated from the source of the seismic event. A large-aperture seismic array may be useful to observe changes in polarity and amplitude of the radiated energy. Inversion of seismic moment tensors can provide a way to characterize microseismic events to gain an understanding of stresses and strains in an earth structure, including the orientation and propagation of fractures in the earth structure.
Non-linear event location methods may involve selection and time picking of discreet microseismic arrivals for each of a plurality of seismic detectors and mapping the data collected in this way to visually locate the source of microseismic energy. However, to successfully and accurately locate the microseismic event, the discrete time picks for each seismic detector need to correspond to the same arrival of either a “P” or “S” wave, and the discrete time picks for each seismic detector must measure/be associated with an arrival originating from a unique microseismic event.
Imaging approaches to detection and location of seismic events may involve summing the signals recorded by a seismic array. However, when there are changes in the polarity of the moment tensor, signals having opposing polarity cancel each other out during stacking computations, rather than sum constructively.
In the hydrocarbon industry, feedback on seismic events occurring in the formation can be used to plan various stages of wellsite operations. Feedback in real-time enables operators to intervene, direct or redirect the operations during the process to optimized production results obtained from the process.
Some feedback methods involve processing microseismic event locations by mapping microseismic arrival times and polarization information into three-dimensional space through the use of modeled travel times and/or ray paths. Travel time look-up tables may be generated by modeling for a given velocity model. One mapping method is commonly known as the “Non-Linear Event Location” method. Non-linear event location has also been used to locate macro seismic events such as earthquakes. Additional information on the topic can be found in U.S. Pat. No. 7,391,675 to Drew, incorporated herein in its entirety, and the references listed therein.